Quick, name the top-selling kids artist of 2010. Miley Cyrus? The Jonas Brothers? Think again. According to Billboard, the Kidz Bop Kids came in No. 1 last year.

In the past two years, Kidz Bop’s music sales have soared 42%, with a fresh compilation featuring clean versions of hit songs covered by anonymous young vocalists dropping every six months. “Kidz Bop 18″ was the No. 1 kids album of 2010, while the 19th and 20th editions both debuted at No. 2 on the Billboard Top 200 chart.

The series is the brainchild of Razor & Tie co-founders Cliff Chenfeld and Craig Balsam. A decade ago, as dads stuck attending birthday parties with their children, they witnessed that 5- to 10-year-olds were an under-served market.

Chenfeld describes their audience as “kids who are too old for Barney, but too young for Britney,” explaining, “We felt there wasn’t a lot of music for kids who had outgrown preschool music but whose parents were concerned that a lot of the kids music out there was inappropriate for them.”

Eschewing the idea of star power, the duo thought young listeners might find regular kids more relatable than polished professionals. “There’s an accessibility in the Kidz Bop world and an aspirational quality that probably doesn’t apply when they’re listening to Justin Bieber or Radio Disney,” Chenfeld says. “Listening to someone like you, I think that’s very appealing to kids.”

Kidz Bop’s brand extensions are proving to be phenomena in their own right. Social networking site KidzBop.com is a kid-friendly, Coppa/Caru-compliant mashup of Facebook and YouTube with more than a million registered users. Through KidzBop.com, talent competition Kidz Star USA offers contestants 15 and younger the chance at an RCA/Jive recording contract. Meanwhile, Kidz Bop is amassing partnerships, including a Web-based reality show with Endemol, a book series with Adams Media and a toy line with Imperial Toy.